1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiple Access communication system (OFDMA communication system) employing an orthogonal frequency division multiple access scheme, and more particularly to a method and an apparatus for generating a preamble sequence for an adaptive antenna system (AAS) in an OFDMA communication system.
2. Description of the Related Art
Fourth generation (4G) communication systems (i.e., the next generation communication system) are being designed to provide users with services having various qualities of service (QoS) and supporting a transmission speed of about 100 Mbps. Particularly, in current 4G communication systems, research is being actively pursued to support a high speed service by ensuring mobility and QoS in broadband wireless access communication systems, such as a local area network (LAN) system and a metropolitan area network (MAN) system. A representative communication system currently under research is an Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system.
The IEEE 802.16 communication system employs an orthogonal frequency division multiplexing and orthogonal frequency division multiplexing access (OFDM/OFDMA) scheme in order to enable a physical channel of the wireless MAN system to support a broadband transmission network. The IEEE 802.16 communication system employs an orthogonal frequency division multiplexing scheme ‘OFDM’ and an orthogonal frequency division multiplexing access scheme (OFDMA) scheme in order to enable a physical channel of the wireless MAN system to support a broadband transmission network. The IEEE 802.16 communication system includes an IEEE 802.16d communication system and an IEEE 802.16e communication system. The IEEE 802.16d communication system considers only one state in which a subscriber station (SS) is currently motionless (i.e., one state in which the mobility of the SS is not entirely considered), and a signal cell structure. In contrast to the IEEE 802.16d communication system, the IEEE 802.16e communication system considers the mobility of an SS. Herein, for the convenience of explanation, a subscriber station with mobility is called a mobile subscriber station (MSS).
Meanwhile, the IEEE 802.16e communication system uses a multi-antenna scheme to expand the communication system's cell service area and also employs a space division multiple access (SDMA) scheme to increase the communication system's capacity. In order to employ the SDMA scheme, it is necessary to design preambles so that the channel quality information (CQI) of each MSS can be precisely measured. A base station minimizes interference between beams by using the correlation between the preambles and generates a precise beam with a channel state estimated according to each MSS, so that the base station can correctly decode data by preventing a signal of one MSS from interfering with signals of other MSSs.
Hereinafter, the construction of the IEEE 802.16e communication system employing the SDMA scheme will be described with reference to FIG. 1.
FIG. 1 is a diagram schematically illustrating the construction of a cellular system which employs a general SDMA scheme.
A base station 101 assigns the same frequency resource and time resource to MSSs (not shown) different from each other so that the MSSs may simultaneously use the same frequency resources and time resources in a first spatial channel transmitted through a first beam 102 and a second spatial channel transmitted through a second beam 103. In order to assign the same frequency resources and time resources to multiple MSSs as described above, it is necessary for the base station to form a plurality of beams spatially separated from each other.
Meanwhile, in order to form the beams as described above in downlinks, correct state information of uplink channels is required. Therefore, in a typical IEEE 802.16 OFDMA communication system, an AAS preamble sequence to support the AAS is added to each of downlink and uplink frames to be transmitted, so that it is possible to determine correct state information of downlink and uplink channels. FIG. 2 is a diagram schematically illustrating a frame structure of a general IEEE 802.16e OFDMA communication system.
The frame of the IEEE 802.16e OFDMA communication system is divided into a downlink frame 201 and an uplink frame 202. The downlink frame 201 includes a downlink preamble section, a frame control header (FCH) section, a downlink MAP (DL-MAP) section, an uplink MAP (UL-MAP) section, a plurality of AAS preamble sections, and a plurality of downlink burst (DL burst) sections (e.g., downlink burst section #1, downlink burst section #2, downlink burst section #3 and downlink burst section #4).
An downlink preamble section is an area for transmitting a synchronizing signal for synchronization acquisition between a transmitter and receiver (e.g., a base station and an MSS), that is, an area for transmitting a downlink preamble sequence. The FCH section is an area for transmitting basic information about a sub-channel, ranging, a modulation scheme, etc. The DL-MAP section is an area for transmitting a DL-MAP message and the UL-MAP section is an area for transmitting an UL-MAP message. Herein, information elements (IEs) included in the DL-MAP message and UL-MAP message have no direct relation to the present invention, so detailed description thereof is omitted. The AAS preamble sections are areas for transmitting a downlink AAS preamble sequence to support the AAS, and the downlink burst sections are areas for transmitting downlink data to target the MSSs.
The uplink frame 202 includes a plurality of AAS preamble sections and a plurality of uplink burst (UL burst) sections, that is, uplink burst section #1, uplink burst section #2, uplink burst section #3 and uplink burst section #4.
The AAS preamble sections in the uplink frame 202 are areas for transmitting an uplink AAS preamble sequence to support the AAS, and the uplink burst sections are areas for transmitting uplink data of the MSSs to target the base station.
Meanwhile, as shown in FIG. 2, an AAS preamble is transmitted at a stage previous to each of the DL bursts and UL bursts. The base station estimates an uplink channel state by using the downlink AAS preamble sequence and generates a downlink beam corresponding to the estimated uplink channel state.
However, in the current IEEE 802.16e communication system standard, distinct AAS preamble sequences for spatial channels different from each other (i.e., for beams different from each other) are not defined. Therefore, when a plurality of MSSs located at positions different from each other, each transmit the same preamble sequence by the same sub-carrier at the same time in an uplink, it is impossible for the base station to estimate a channel for each MSSs and generate a corresponding beam. That is, the conventional AAS preamble sequence cannot support the SDMA scheme in the OFDMA communication system. Therefore, it is desirable to develop a methodin which distinct AAS preamble sequences are assigned to spatial channels different from each other and thus each receiving side of the AAS preamble sequences can distinguish the spatial channels to generate a beam.